(resending to café, turns out I wasn't subbed from this address.)
Hi Magicloud,
This is correct; because you've hidden the type-variables away by universally quantifying them, there's no more level of specificity you can get back *out* of them than just "some kind of Map" (Job = M.Map k b, where k ≠ k0, b ≠ b0).
If you have a Job type which can store *any* kind of Map (forall k a. Job (Map k a)), then that means you could have a Job with a Map Int Bool, and a Job with a Map String (Float -> Float), and they'd both have the same type "Job". You can't do anything with the values within, because you're being too permissive about what a Job is.
You may want "data Job k a = Job (Map k a)", *or* if you do actually use one kind of Map only, then why not "data Job = Job (Map Int String)" (substituting your real types for Int and String). In this case, you could also consider using newtype ("newtype Job = Job { getJob :: Map Int String }") to provide the guarantee that you're getting a Job (and not any Map Int String) without performance loss.
Let me know if I've been more confusing than helpful;
Arlen
On Thursday, 14 June 2012 at 1:16 PM, Magicloud Magiclouds wrote:
> Hi there,
> Thanks for the reply. To be clear, all I want is to "avoid having to
> type type variables all over the place". What should I do? My original
> code with RankNTypes and ImpredicativeTypes does not work....
>> The "type Job = forall k a. M.Map k a" works now. But function uses
> it does not. Compiler complains about "Couldn't match expected type
> `Job' with actual type `M.Map k0 b0'".
>> On Wed, Jun 13, 2012 at 9:15 PM, Daniel Peebles <pumpkingod at gmail.com (mailto:pumpkingod at gmail.com)> wrote:
> That doesn't require existential quantification, but it'll need Rank-2 typesif you ever do anything with Job. Unfortunately, a universally quantifiedJob like what you wrote (or what Magicloud seems to want) is only inhabitedby the empty Map.
> >
> An existentially quantified Job, as you might get with
> >
> data Job = forall k a. Job (Map k a)
> >
> does let you wrap up any Map containing anything in it, but unfortunatelythe only thing you can do with that map afterwards is ask for "structural"properties about it, like whether it's empty or how many elements it has init. You could ask to enumerate the elements in it, but you wouldn't be ableto touch any of them because you wouldn't know what their types were.
> >
> So I'm not really sure how to interpret the question. Was the goal to have aheterogeneous Map, maybe? Or just to avoid having to type type variables allover the place? Both of those are possible but require a bit moresophistication with types.
> >
> -Dan
> >
> >
> On Wed, Jun 13, 2012 at 7:32 AM, Ismael Figueroa Palet<ifigueroap at gmail.com (mailto:ifigueroap at gmail.com)> wrote:
> > >
> Do you want to hide the specific types of the job? Presumably to thendefine a type JobList = [Job] ?You can do that with the ExistentialQuantification extension.
> > >
> type Job = forall k a. Map k atype JobList = [Job]
> > >
> ??Note you can't unpack the types k a once you have hidden them. But thetypechecker can use it to ensure some static property.Also you could use unsafeCoerce to do some casts, but *only if you are*sure* that things will go OK*.
> > >
> > >
> 2012/6/13 Magicloud Magiclouds <magicloud.magiclouds at gmail.com (mailto:magicloud.magiclouds at gmail.com)>
> > > >
> Hi,I've forgotten this.This is OK:type Job k a = Map k aAnd this is OK:{-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms?type Job = forall a. forall k. Map k a
> > > >
> Then how to write it like this?type Job = Map k a--竹密岂妨流水过山高哪阻野云飞
> > > >
> And for G+, please use magiclouds#gmail.com (http://gmail.com).
> > > >
> _______________________________________________Haskell-Cafe mailing listHaskell-Cafe at haskell.org (mailto:Haskell-Cafe at haskell.org)
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> > >
> > >
> > >
> > >
> > >
> > >
> > >
> --Ismael
> > >
> > >
> _______________________________________________Haskell-Cafe mailing listHaskell-Cafe at haskell.org (mailto:Haskell-Cafe at haskell.org)
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>>>>>>>> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>> And for G+, please use magiclouds#gmail.com (http://gmail.com).
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